servers.List() pagination is extremely inefficient #3867

What happened?

I was investigating remarkably bad performance in a component that runs github.com/gophercloud/gophercloud/v2/openstack/compute/v2/servers.List(), and the profiling showed that the problem is with how pagination is implemented inside of it. It unmarshals the response body no less than four times:

1. once into map[string]any during pagination.PageResultFrom() for use in generic NextPageURL/IsEmpty implementations (which do not end up being used)
2. once in ServersPage's custom NextPageURL implementation
3. once in ServersPage's custom IsEmpty implementation
4. once in ServersPage.Extract() to get the actual []servers.Server

While this is clearly suboptimal, usually it's not all that bad because most JSON responses from OpenStack are not all that large. But servers.List() contains server objects which not only have an extremely large number of attributes, some of those attributes are also genuinely huge (esp. userdata).

For my own usecase, I mitigated the problem by using a custom pagination implementation that bypasses the standard types from package pagination entirely. I don't think that it will be possible for the standard pagination implementation to be as efficient (because of how func Extract always does a separate unmarshal), but it might be possible to avoid the three separate unmarshals for NextPageURL and IsEmpty.

Version

v2 (current)